1. Field of the Invention
The field of the present invention is that of turbine engines and, more particularly, that of their accessory housing, known generally under the name accessory relay or accessory gearbox (AGB).
2. Description of the Related Art
Some of the power generated by a turbojet engine is used in order to supply power to various items of equipment of the turbojet engine. This power is taken mechanically from the shaft of the high-pressure (HP) body of the turbojet engine by a power take-off shaft which drives an input shaft in the accessory gearbox. This housing contains a number of gears linked to equipment or accessories, such as for example an electric generator, a starter, an alternator, hydraulic pumps, fuel pumps or oil pumps, etc. These various accessories are driven mechanically by the input shaft in the AGB which transmits to each of them, via the gears of the AGB, some of the power taken from the HP shaft.
The AGB generally comprises a housing manufactured by casting an aluminium alloy, which forms a compartment, closed above and below by two substantially parallel walls, in which are disposed the gear wheels which drive the accessories. Each gear wheel comprises a shaft which is capable of receiving the drive shaft of an accessory and one of them is likewise linked to the input shaft in the AGB. The different accessories driven by the AGB are mounted directly on the cast housing, the drive shafts of these accessories passing through cutouts made in one of the walls of this housing in order to drive the accessories and being dimensioned to enable mounting of the corresponding gear wheel inside the housing. The gear wheels are positioned in the cast housing by being supported, on the one hand, by the wall of the housing which is opposite the cutout for passage of the drive shaft thereof and, on the other hand, by a removable cover which is attached to the cut-out wall of the housing. As a general rule, without this configuration being mandatory, the shaft of the gear wheel is supported, on the one hand, by the outer race of a roller bearing attached to the uncut base wall of the housing and, on the other hand, by the outer race of a ball bearing attached to the cover associated with said accessory.
Such configurations are described, for example, in the patent applications FR 2556413 or DE 954481 which relate to accessory gearboxes in which the drive shafts of the accessories are supported by bearings of which the outer races are supported by the gearbox housing.
The metal generally used in aeronautics to produce the housing of the accessory gearbox is an aluminium alloy known by the AFNOR designation AS7G06 (or A357 as its trade name), which has a low mass and exhibits useful mechanical characteristics. The cover, for its part, is made of another aluminium alloy, with the trade name AU2GN (i.e. AlCu2MgNi or 2618A as its AFNOR designation). Said alloy has inter alia the advantage of retaining good mechanical resistance even if the temperature increases, up to a value of approximately 150° C.
On the other hand, the alloy AS7G06 has the drawback that its hardness declines with temperature and vibration of the pins which attach the outer race of a roller bearing have been noted on engines in use. The incidents which occur result in fact from the heating of the casing due to an accessory, such as for example a permanent magnet alternator which supplies a substantial heat flow and heats the aluminium alloy to the point where it reaches a critical temperature, or also result from a high level of vibration which is generated by this same accessory or by the input shaft in the AGB.